Merge #4650

4650: Lower chance of losing written files by syncing them to disk on close, my try r=def- a=heinrich5991

Not sure if it helps, but now it only syncs the config file.

Would supersede #4407.

## Checklist

- [x] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [x] Written a unit test if it works standalone, system.c especially
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)


Co-authored-by: def <dennis@felsin9.de>
Co-authored-by: heinrich5991 <heinrich5991@gmail.com>

src/base/system.cpp

@@ -69,6 +69,7 @@
 #include <direct.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <io.h>
 #include <process.h>
 #include <share.h>
 #include <shellapi.h>
@@ -469,6 +470,19 @@ int io_flush(IOHANDLE io)
 	return fflush((FILE *)io);
 }
 
+int io_sync(IOHANDLE io)
+{
+	if(io_flush(io))
+	{
+		return 1;
+	}
+#if defined(CONF_FAMILY_WINDOWS)
+	return FlushFileBuffers((HANDLE)_get_osfhandle(_fileno((FILE *)io))) == 0;
+#else
+	return fsync(fileno((FILE *)io)) != 0;
+#endif
+}
+
 #define ASYNC_BUFSIZE 8 * 1024
 #define ASYNC_LOCAL_BUFSIZE 64 * 1024
 

src/base/system.h

@@ -303,6 +303,18 @@ int io_close(IOHANDLE io);
 */
 int io_flush(IOHANDLE io);
 
+/*
+	Function: io_sync
+		Synchronize file changes to disk.
+
+	Parameters:
+		io - Handle to the file.
+
+	Returns:
+		Returns 0 on success.
+*/
+int io_sync(IOHANDLE io);
+
 /*
 	Function: io_error
 		Checks whether an error occurred during I/O with the file.

src/engine/shared/config.cpp

@@ -110,6 +110,11 @@ bool CConfigManager::Save()
 	for(int i = 0; i < m_NumCallbacks; i++)
 		m_aCallbacks[i].m_pfnFunc(this, m_aCallbacks[i].m_pUserData);
 
+	if(io_sync(m_ConfigFile) != 0)
+	{
+		m_Failed = true;
+	}
+
 	if(io_close(m_ConfigFile) != 0)
 		m_Failed = true;
 

src/test/io.cpp

@@ -68,3 +68,13 @@ TEST(Io, CurrentExe)
 	EXPECT_GE(io_length(CurrentExe), 1024);
 	io_close(CurrentExe);
 }
+TEST(Io, SyncWorks)
+{
+	CTestInfo Info;
+	IOHANDLE File = io_open(Info.m_aFilename, IOFLAG_WRITE);
+	ASSERT_TRUE(File);
+	EXPECT_EQ(io_write(File, "abc\n", 4), 4);
+	EXPECT_FALSE(io_sync(File));
+	EXPECT_FALSE(io_close(File));
+	EXPECT_FALSE(fs_remove(Info.m_aFilename));
+}